The present invention relates to a process for isomerizing olefinically unsaturated alcohols in an oxygenous atmosphere over a supported catalyst. The invention further relates to the use of supported catalysts for isomerizing olefinically unsaturated alcohols.
The isomerization of olefinically unsaturated alcohols, for example of 3-methylbut-3-en-1-ol (isoprenol) to 3-methylbut-2-en-1-ol (prenol), in liquid phase has already been described. In all cases, the conversion is performed under a gentle stream of hydrogen to increase the catalyst lifetime (S. B. Kogan, M. Kalya, N. Froumin, Appl. Catal. A: General 2006, 27, 231-236). According to the information in this document, the noble metal catalysts described are deactivated particularly rapidly and is irreversibly in the absence of hydrogen. Even under optimal conditions, according to this document, the conversion achieved with these catalysts is relatively low when high selectivities in the direction of prenol are to be achieved. At low conversions, the unconverted isoprenol has to be removed and reused for economic operation of the process. The use of hydrogen in the isomerization of isoprenol is often disadvantageous, since side reactions such as the hydrogenation of the double bonds to give the saturated system are observed particularly over silica-supported noble metal catalysts. Attempts are made to avoid this by the expensive stabilization of the cationic palladium sites by doping with selenium or cerium.
U.S. Pat. No. 4,117,016 describes, inter alia, the isomerization of unsaturated alcohols in the liquid phase in the presence of catalytic amounts of a homogeneous ruthenium-hydrido complex in the presence of a ligand, and explicitly rules out the presence even of traces of oxygen, in order to ensure catalyst stability. The isomerization of isoprenol to prenol under nitrogen under the conditions specified in this document proceeds only in low yields which are not of economic interest.
WO2008037693 A1 describes, inter alia, the isomerization of 3-methyl-3-buten-1-ol (isoprenol) over a heterogeneous noble metal catalyst to give 3-methyl-2-buten-1-ol (prenol) in the presence of hydrogen. The trimetallic catalyst consists of Pd, Se and Te immobilized on silica. Here, overhydrogenation in the hydrogen atmosphere results in the formation of up to 2.5% isoamyl alcohol. The removal thereof is costly.
DE-A-1901709 describes a process for preparing buten-2-ols from buten-1-ols. The catalysts used are elemental Pd or Pd compounds in a hydrogen atmosphere. In the case of use of pure Pd in the presence of hydrogen, however, the majority of the double bonds of the starting materials are hydrogenated to form the corresponding saturated compound.
The hydrogenation of the double bond is undesired since there are only small boiling point differences between unconverted starting material and hydrogenation product for some butenols. For instance, the boiling point of 3-methylbut-3-en-1-ol is 131.5° C. (101 325 Pa or 1020 mbar), and that of the corresponding hydrogenation product is 130.9° C. (101 325 Pa or 1020 mbar).
DE-A 2751766 and the parallel U.S. Pat. No. 4,310,709 disclose that, in the case of isomerization of 3-buten-2-ols to the corresponding 2-buten-1-ols in the presence of a heterogeneous catalyst based on Pd and Se on carbon in the presence of hydrogen, high proportions of low boilers such as isoprene and butenes are formed in the manner of a retro-Prins reaction.
EP-A 841090 discloses that the isomerization of 3-buten-1-ols to 2-buten-1-ols in the presence of hydrogen succeeds only with low formation of low boilers or hydrogenation products when the heterogeneous Pd catalyst has been doped with 0.001-0.2% by weight of Se, Te or with a mixture of both metals. Thus, selectivities of up to 94% can be achieved at a conversion of 55%.
Can. J. Chem. 1968, 46, 2225-2232 discloses the thermal isomerization of unsaturated alcohols without catalyst. At the high temperatures required, partial resinification of the starting compounds is observed.
According to the teaching of JP-A-8268939, prenol is isomerized to isoprenol in the gas phase in the presence of catalytic amounts of MgO. The temperatures are very high at 150-300° C. and lead to the decomposition of the labile reactants and products. For safety reasons, the reaction is explicitly performed under protective gas (nitrogen). It is possible to achieve selectivities of up to 98% at conversions of 64%.
In spite of several known processes for isomerizing unsaturated alcohols, more particularly including isoprenol to prenol, there was therefore a need to provide a corresponding process which is firstly performable in a simple manner in terms of process technology and secondly delivers good conversions and selectivities in the direction of the desired products.